Crack-growth information is important in estimating fatigue life in metallic structures, such as employed in aircraft fabrication. With respect to the latter-mentioned structures, the interaction between existing residual stresses in aluminum alloy sheet stock and residual stresses induced by cold working holes is difficult to evaluate analytically, mainly because existing residual stresses are not clearly defined. Accordingly, there is a need to experimentally evaluate fatigue specimens under cyclic loading so that critical fatigue life parameters, such as intensity factor K.sub.I can be determined during crack growth. This would better enable inspectors to determine whether supplied stock sheet material has been properly manufactured and, in the event holes are formed in such material and cold worked, the success of such cold working can be determined
The method of "caustics" or shadow photography has been used in recent years for the evaluation of strain fields near crack tips in laboratory work. These methods have generally been used in connection with brittle materials which develop short cracks after cyclic loading and then quickly fail. Data evaluation of crack tips in such material have been analyzed by investigators. To date, there has been no known attempted use of this method for the evaluation of residual stresses around holes formed in more ductile sheet materials where cracks can form along a fairly long length before structural failure. In a number of industries, such as the aircraft fabrication industries, the fatigue life before such structural failure is of obvious great concern.